Effect of local equivalent ratio on spark ignition characteristics in a kerosene scramjet combustor at Mach 4 flight condition

The effect of local equivalent ratio on spark ignition characteristics is investigated in this article. By adapting combined injection, the fuel-lean and fuel-rich local equivalent ratio are formed inside ignition cavity, and the processes of initial flame generation and local flame self-sustain have been analyzed. The experimental result shows that, the characteristic of spark ignition is distinct in fuel-lean and fuel-rich local equivalence ratio. When the local equivalence ratio is relatively low, few initial flame kernels can be generated after the spark discharges, resulting in potential ignition delays or failures. Alternatively, when the local equivalence ratio is relatively high, initial flame can be generated frequently after spark discharge. However, the fuel-rich environment inside ignition cavity hinders flame stabilization, causing rapid flame extinguishment and continuous pressure oscillations inside ignition cavity. The spatial distribution of the fuel spray significantly impacts the characteristics and intensity of local flame. During the experiments with double injections, the spray exhibits a dense concentration near the shear layer of cavity T1. This dense spray weakens the local flame, suppressing the formation of a tail flame and preventing its propagation. In contrast, the experiments utilizing single injection demonstrate a sparse distribution of spray near the shear layer of cavity T1. Consequently, the tail flame can be formed at the rear edge of cavity T1 more frequently. These local flames possess the potential to develop into downstream flames through the spreading of the tail flame.